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F.1 It seems that there are always conflicting stories about climate change appearing in the media. Is there no agreement among scientists about climate change?

Response: The vast majority of scientists studying climate change agree that the basis for concern is scientifically sound. Although there may be individual scientists who disagree with this consensus and whose opinions may be featured in stories in the media, it is important to consider what scientific support there is for their arguments. The best approach is to refer to the peer reviewed published literature, since a key test for credibility is whether a paper has successfully gone through a process of peer review (review by scientists with relevant expertise).

Explanation: Each year, there are several thousand new scientific papers published in peer-reviewed journals on topics related to climate change. Each paper adds a small increment to the large body of knowledge already available. Since the global climate system is very complex, these papers involve many different scientific disciplines, and are focused on a broad range of processes and causes of climate change. Some processes involve negative feedbacks that reduce the initial climate response; others involve positive feedbacks that amplify it. Some causes for change, whether natural or human, tend to cool climate, while others induce warming. Each new scientific paper has to be put in context with all that has preceded it. The best approach for evaluating the implications of new science is through periodic assessments of the scientific literature. Such assessments will focus on recent findings and will place them in context with what was known and understood about an issue previously. The best known process for assessing the scientific literature on climate change is that undertaken by the Intergovernmental Panel on Climate Change (IPCC).

Sometimes, a new scientific paper may be published that appears to contradict established wisdom. Such a result would be intrinsically of special interest to any scientist. Normal scientific process would see such results get intense scrutiny by other scientists and there would be further research undertaken aimed at replicating the results. This process of verifying and replicating new results takes time and sometimes the concurrent debate among scientists is mistaken by non-scientists to mean there is some important disagreement among scientists. Generally, this is not the case. In the field of climate change, there is certainly debate about the details, and the need to better understand the particulars of how the climate system will respond to increases in greenhouse gases is what drives ongoing research efforts. However, the background science upon which concern about climate change is based is much less controversial. A good summary of such background information was recently provided in the Fourth Assessment Report released by the Intergovernmental Panel on Climate Change in 2007.

F.2 I understand there are thousands of scientists who argue that we know too little about climate change, and that it is therefore premature to respond. Who are these dissenters and are they credible?

Response: The numbers often quoted with respect to dissenting scientists are not supported by the published literature in scientific fields related to the study of climate change. Very few of the dissenters publish scientific research papers in those journals in which the majority of papers on climate change are published, that is, in the forum where scientific discussions should occur. It is important to pay attention to the field of work and level of expertise of those acting as sources of information in the climate change discussion. Review by scientists with relevant expertise is a good measure of credibility.

Explanation: Over time, as scientific evidence in support of human-induced climate change has grown, the arguments put forth by dissenters have changed. While some dissenters may still argue that the global warming trend is not real and that the human influence on climate is not yet apparent - arguments easily refuted by published peer-reviewed science - many now focus their arguments on the projected rate and consequences of future climate change which they claim are exaggerated. There are some scientists who do argue that the science about climate change is uncertain enough that much more research is needed before measures to substantially reduce greenhouse gas emissions are taken. Whether or not to act in the face of uncertainty is not a scientific issue, but rather a policy decision, one which requires a risk management approach. Scientists can, however, provide advice about the nature of the risks and impacts that may be associated with different magnitudes of global warming (and related climatic changes). Most of the dissenters with credible scientific backgrounds generally agree with the fundamental science underlying the concern about climate change.

F.3 With so much uncertainty about future climate change, why don't we hold off on any reductions in CO2 emissions until we are better able to better predict what will happen?

Response: The scientific concern about climate change is well-founded. Many of the remaining uncertainties are related to the details of the consequences of global climate change. Scientists are in general confident that the basis for concern about climate change is scientifically sound, that humans are largely responsible for the change in climate during the past 50 years, that the risks of danger due to projected changes in climate are real and significant. Unrestrained increases in atmospheric greenhouse gas concentrations would be catastrophic. The only way to stabilize atmospheric greenhouse gas concentrations is to reduce global emissions of these gases. When and how this should be done is a policy decision but the fact that the climate system responds slowly to changes in emissions makes it prudent that we begin precautionary action now.

Explanation: While there is uncertainty as to the magnitude and rate of climate change, particularly at the regional level, scientists generally agree that rates of change over the next century will almost certainly be greater than anything experienced on Earth during the past 10,000 years. More significantly, the change could be as large as that experienced during the deglaciation at the end of the last ice age, but more than 10 times as fast - an experiment on the climate system with risky consequences. Furthermore, because of the long delay in the response of the climate system to changes in radiative forcing, by the time all the evidence is in it may be too late to avoid significant danger. Given that there is considerable inertia in both society and the global climate system - the former to changes in cultural behaviour and in technological restructuring, the latter to changes in radiative forcing - early action is prudent. The scientific community has recommended precautionary action that will reduce the risks by slowing down the potential rate of climate change.

F.4 Is it too late to stop climate change?

Response: Scientists agree that the current warming trend cannot be stopped or reversed. However, it can be slowed down to allow biological systems and human society more time to adapt.

Explanation: There are two reasons why further climate change is already unavoidable. First, there is a lot of inertia in the climate system mainly because of the slow response of oceans. This means that the oceans have not yet fully warmed to the level they will eventually reach under current greenhouse gas concentrations, and are still somewhat cooling off the atmosphere. Even the atmospheric response to current greenhouse gas concentrations is not yet fully realized. If all emissions stopped today, the oceans would continue to warm for a number of decades until they finally reached a new equilibrium. Second, while global emissions of greenhouse gases can be slowed down, it will take time for transition from a fossil-fuel-based global economy to alternatives. Further emissions and incremental warming are therefore unavoidable. The fact that some additional climate change is unavoidable means that adapting to climate change is a necessity. Mitigative actions are likewise essential, to slow down and eventually stop the rise in global emissions. As long as the atmospheric concentration of greenhouse gases increases, there will continue to be a 'positive forcing' - a warming effect - on climate. To stabilize atmospheric concentrations of greenhouse gases will require a reduction of global greenhouse gas emissions.

F.5 Isn’t it more important to tackle air pollution first, since the risks it poses to our health are more immediate?

Response: Air pollution issues such as smog and acid rain are indeed of immediate concern, while the more serious impacts of climate change are further in the future. However, there are multiple and complex linkages between climate change, smog and other local air pollution concerns that suggest that there are many benefits to addressing these issues at the same time. First, the key ingredients of smog also have important roles in the climate system. O3, for example, is a greenhouse gas that contributes to global warming. Likewise, sooty aerosols absorb sunlight and add to local warming. In contrast, sulphate aerosols reflect sunlight and alter local cloud properties, both of which tend to cool the climate. Most importantly, all of these substances are directly or indirectly released through many of the same human activities that release long-lived greenhouse gases such as CO2. Second, climate change can affect O3 chemistry, since chemical reactions are influenced by both atmospheric temperatures and the amount of sunlight. Ecosystems and societies that are affected by smog may already be stressed due to climate change, and the combined effects may lower the critical thresholds for catastrophic loss. For example, an elderly person already stressed by high temperatures during a heat wave could suffer additional respiratory stress in association with high smog concentrations.

Explanation: The combustion of fossil fuels and other industrial sources of greenhouse gas emissions are also important sources of the precursors to tropospheric O3 and the particulate matter that contribute to local air pollution, particularly smog. Furthermore, while O3 is a particularly harmful component of smog, it is also a greenhouse gas that is estimated to be the third largest contributor to historical enhancement of the greenhouse effect. Sooty aerosols within smog absorb incoming solar radiation and thereby contribute to global warming. Reducing these aerosol emissions would both reduce urban smog and global warming. On the other hand, some of the other particulates within smog, particularly sulphate aerosols, reflect sunlight and alter cloud properties. These effects tend to cool climate. Measures to reduce their emissions to improve local air quality tend to increase warming.

Since sunlight and air temperatures are important factors in surface O3 chemistry, rising temperatures and changes in cloud cover due to climate change will affect O3 chemistry, and hence smog intensity. Furthermore, changes in wind direction and in precipitation frequency and intensity will affect O3 transport and the efficiency of particulate removal by precipitation.

Finally, smog causes serious health impacts, including significant increases in mortality from cardiovascular and cardio-pulmonary diseases as well as cancer. It also damages vegetation. In general, the combined effects of multiple stresses from climate change and air pollution will increase the likelihood of exceeding critical thresholds for ecological stress tolerance, increasing the chance of related morbidity and mortality, particularly amongst the most vulnerable.

All of these linkages between climate change and air pollution speak to the need for a coordinated response to 'atmospheric change' which would take into consideration how actions to improve any one environmental problem would affect other issues. Where there are common sources of multiple pollutants, such as the combustion of fossil fuels, there are multiple benefits to reducing such activities.